A 40-year-old woman presents to the clinic for evaluation of symptoms of light-headedness associated with sweating, palpitations, and hunger. The symptoms are most pronounced whenever she misses a meal.On physical examination, her vital signs are normal, as is the heart, lungs, and abdominal examination. Her past medical history is negative and she is not taking any medications. During one such episode, while in hospital, her blood glucose level was 30 mg/dL and the symptoms resolved with drinking some juice. Which of the following is the most likely diagnosis?